It can be done but you'll also need the ECU, wiring and all ancillary items.
Question: why put a 2nd gen 3SGTE into a GT as opposed to a better equipped GTS?
(I.E. drum brakes vs disks; power seat; 5-bolt wheels; bigger tires; etc.)
With all costs and time factored in, you might want to just look for an 88-89 All-Trac. They can be found for anywhere from $1500 - $3500, depending on condition and mods. That way, you have an economical GT for DD and another with 3SGTE for performance and fun.
Just my 2 cents....
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